The present invention generally relates to what will be referred to herein as “overtemp circuits”; namely, circuits included in electrical designs which remove or reduce power supplied to one or more components when a temperature (e.g., junction temperature, ambient temperature) exceeds a threshold (e.g., indicative of a thermal runaway event) or indicates an impending thermal runaway event. More specifically, the present invention relates to overtemp circuits in LED lighting systems, and apparatus, means, and methods for preventing thermal runaway events or mitigating undesirable lighting effects that occur after the thermal runaway event/temperature threshold issue is resolved and power is returned to the one or more components of the lighting system.
It is well known that in recent years the reduction in cost and increase in luminous efficacy (lm/W) of LEDs has permitted their use beyond novelty and general purpose lighting and into areas of more specialized lighting. For specialized lighting applications such as wide area or sports lighting, often a large number of LEDs (e.g., many hundreds for a single tennis court) are required to provide uniform lighting that meets minimum requirements—see, e.g., Illuminating Engineering Society (IES) RP-06-01 Recommended Practice for Sports and Recreational Area Lighting for examples of such lighting requirements. As is also well known in the art, high luminous efficacy—a primary selling point of LEDs—is only realized if LED junction temperature is kept low. Thus, it stands to reason that using many hundreds (if not thousands) of LEDs so to adequately light a sports field to one or more standards (as dictated by governing bodies, municipalities, or otherwise) cannot be done in a cost-effective manner unless measures are taken to control the temperature of said LEDs.
In the art of LED wide area or sports lighting there currently exist two approaches to controlling temperature: passive and active cooling. Passive cooling techniques are generally defined as means which do not require external forces or, to some extent, moving parts. An external fixture housing which is designed to promote airflow, formed from thermally conductive material, and includes a number of heat fins to increase surface area is an example of a passive cooling technique in LED lighting design; another is the inclusion of heat pipes or thermosyphons such as is discussed in U.S. Provisional Patent Application Ser. No. 62/118,675 incorporated by reference herein in its entirety. Active cooling techniques in the current art of LED lighting design typically center around forced air or fluid in, through, around, or generally proximate heat sources (e.g., LEDs); some examples are described in U.S. Pat. Nos. 8,651,704 and 9,028,115 which are incorporated by reference herein in their entirety. It is generally understood that active cooling techniques are more aggressive and remove or redistribute heat from an LED lighting system more effectively than passive cooling techniques.
If passive or active cooling techniques fail (e.g., power to a fan is disabled), one would expect the temperature of the LEDs to increase and efficacy to decrease. If said cooling techniques are applied system-wide, the temperature of other components (e.g., drivers) may increase in commensurate fashion upon such a failure. An increase in temperature of an LED lighting system, left unmitigated, could reduce cost effectiveness and damage parts.
It is logical, then, that if such a thermal runaway event occurred—as it will be called herein—a potential solution would be to temporarily reduce or terminate power to the LEDs and/or other temperature sensitive components until the situation is resolved and components cool (e.g., via natural convection). One can think of such a solution as similar to GFCI circuits common in other areas of electrical design; a safety feature that only triggers in extreme events or in anticipation of an extreme event. However, for specialty LED lighting applications such as wide area and sports herein lies a problem—the high voltages required for the large number of LEDs prevents implementation of a traditional overtemp circuit. There are no high voltage (e.g., 1000V) solutions to thermistors or bi-metallic switches that would permit detection of a thermal runaway event and act to open a circuit, thereby terminating power to the LEDs. Thermal fuses are terminal event devices—if a thermal fuse opens a circuit to mitigate a thermal runaway event, that fuse must be replaced before power to the LEDs can be restored. Given that wide area and sports lighting applications typically have the aforementioned hundreds (if not a thousand or more) LEDs mounted several dozens of feet in the air in an environmentally sealed housing, replacing thermal fuses in a luminaire (also referred to herein as a fixture) is highly impractical.
The art is at a loss. LEDs operated in large number under high voltage conditions—such as in wide area or sports lighting applications—are prime candidates for passive and active cooling techniques, and failure of said cooling techniques would likely result in thermal runaway thereby also making such lighting applications prime candidates for overtemp circuits. That being said, there are no adequate overtemp circuits commercially available to address high voltage LED lighting systems. Thus, there is room for improvement in the art.